National Instruments NI 7831R - Manual (Page 43)

© National Instruments Corporation 3-1 NI 7831R User Manual
3
Calibration
Calibration is the process of determining and/or adjusting the accuracy of
an instrument to minimize measurement and output voltage errors. On the
NI 7831R, onboard calibration DACs (CalDACs) correct these errors.
Because the analog circuitry handles calibration, the data read from the
AI channels or written to the AO channels in the FPGA VI is already
calibrated.
Three levels of calibration are available for the NI 7831R to ensure the
accuracy of its analog circuitry. The first level, loading calibration
constants, is the fastest, easiest, and least accurate. The intermediate level,
internal calibration, is the preferred method of assuring accuracy in your
application. The last level, external calibration, is the slowest, most
difficult, and most accurate.
Loading Calibration Constants
The NI 7831R is factory calibrated before shipment at approximately 25 °C
to the levels indicated in Appendix A,
Specifications . The onboard
nonvolatile flash memory stores the calibration constants for the device.
Calibration constants are the values that were written to the CalDACs to
achieve calibration in the factory. The NI 7831R hardware reads these
constants from the flash memory and loads them into the CalDACs at
power-on. This occurs before you load a VI into the FPGA.
Internal Calibration
With internal calibration, the NI 7831R can measure and correct almost all
of its calibration-related errors without any external signal connections.
NI provides software to perform an internal calibration. This internal
calibration process, which generally takes less than two minutes, is the
preferred method of assuring accuracy in your application. Internal
calibration minimizes the effects of any offset and gain drifts, particularly
those due to changes in temperature. During the internal calibration
process, the AI and AO channels are compared to the NI 7831R onboard
voltage reference. The offset and gain errors in the analog circuitry are
calibrated out by adjusting the CalDACs to minimize these errors.